Mercury isotope clocks predict coastal residency and migration timing of hammerhead sharks

1. The management of migratory taxa relies on the knowledge of their movements. Among them, ontogenetic habitat shift, from nurseries to adult habitats, is a behavioral trait shared across marine taxa allowing resource partitioning between life stages and reducing predation risk. As this movement is consistent over time, characterizing its timing is critical to implement efficient management plans, notably in coastal areas to mitigate the impact of fisheries on juvenile stocks.
2. In the Mexican Pacific, habitat use of the smooth hammerhead shark (Sphyrna zygaena) is poorly described, while the species is heavily harvested. Given the large uncertainties associated with the timing of out-migration from coastal nursery grounds to offshore waters prior to reproductive maturity, a more precise assessment of smooth hammerhead shark movements is needed. 
3. Photochemical degradation of mercury imparts mass-independent isotope fractionation (Δ199Hg) which can be used to discriminate between ne..., Sharks were collected along the western coast of Baja California Sur (Mexico). Juvenile smooth hammerhead sharks were caught by gillnets and longlines in 2009 and from 2014 to 2018. Sharks (n=102) were sampled at four locations in artisanal fishing camps: Bahía Tortugas, Las Barrancas, San Lázaro and Punta Lobos. For each individual, approximately 1 g of dorsal white muscle tissue was sampled. Samples were transported on ice, stored at -20°C at the laboratory and ultimately freeze-dried prior to transport and further treatments.
Total Hg concentration (THg) was determined in a 20 mg aliquot of each sample using a Direct Mercury Analyzer (DMA-80, Milestone, USA), by combustion, gold trapping and atomic absorption spectrophotometry detection (Observatoire Midi-Pyrénées, Toulouse, France). THg analytical detection limit was 0.005 μg·g−1 dw (dry weight). Repeated measurements of a tuna flesh homogenate (BCR-464, Institute for Reference Materials and Measurements, 5.24 ± 0.10 μg·g−1 dw) test..., Details of shark samples along with their Hg isotope values (Δ199Hg, δ202Hg, Δ200Hg, Δ201Hg, Δ204Hg) are presented in an EXCEL file (.csv).